glass



Jan. 24, 1956 J. R. GLASS ELECTRODE SYSTEM FOR POLAROGRAPHIC ANALYSIS 2 Sheets-Sheet 1 Filed Oct. 6, 1952 INVENTOR. 121/111 ]1. (i/ass BY MQMZ,

Jan. 24, 1956 J, GLASS ELECTRODE SYSTEM FOR POLAROGRAPHIC ANALYSIS 2 Sheets-Sheet 2 Filed Oct. 6, 1952 Hmeoqwwams p/IEA/YL o(- A/APHTH n AM/NE INVENTOR. Ja /v ,2 64 455 +0.4 H16 APPLIED VOL 7465 .gator and his coworkers.

United States Patent ELECTRODE SYSTEM FOR POLAROGRAPHIC ANALYSIS John R. Glass, Glassboro, N. J., assignor to Socony Mobil Oil Company, Inc., a corporation of New York .Application October 6, 1952, Serial No. 313,301

5 Claims. (Cl. 204-1) This invention relates to an electrode system useful in polarographic analysis or other electrochemical processes trode. The instant invention is concerned, in particular,

with a renewable surface electrode which enables polarographic oxidation curves to be obtained heretofore unattainable with other electrodes.

The polarographic method of chemical analysis was originally developed by Heyrovsky about thirty years ago and has been carried on to a large extent by this investi- The method is based on the interpretation of current-voltage curves which are obtained when solutions of electro-reducible or electrooxidizable substances are electrolyzed in a cell. The cell originally used by Heyrovsky and which has been most extensively employed since inception of the method is the dropping mercury electrode in which the cathode consists of mercury falling dropwise from a very fine bore .capillary tube into a layer of mercury which constitutes the anode. Since such layer has a large area and the currents passed are generally very small, i. e., about ampere, the potential remains almost constant and hence variations in the E. M. F. of the cell are due almost entirely to changes in the cathode potential. From the unique characteristics of the current-voltage curves obtained, both the species and concentration of the electroreducible or electrooxidizable substances present in the solution can be determined.

' The cell may be connected, in series, with a calibrated 'galvanometer to a battery and rheostat, by means of which an E. M. F. from zero up to the maximum E. M. F, of the battery can be applied across the cell. The current-voltage curves may thus be obtained manually by gradually increasing the applied E. M. F. and noting the current indicated by the galvanometer. More conveniently, changes in the current are measured directly by comparison with a standard electrode in a self-registering apparatus, known as the polarograph, in which the 'E. M. F. applied between the two mercury electrodes is 'the cell until the decomposition potential is reached.

When the decomposition potential is exceeded, continuous electrolysis takes place with a sharp rise in the current. The current, however,,does not increase indefinitely with 2,732,335 Patented Jam. 24, 1956 increasing applied E. M. F. after the decomposition potential is exceeded, but gradually approaches a limiting value. Other factors being constant, the limiting current is directly proportional to the concentration of the electroreducible or electrooxidizable substance in solution, thereby atlording a quantitative analysis of such solution.

The dropping mercury electrode has been used in many investigations of cathodic phenomena, including studies of deposition potentials of ions, hydrogen ion concentration and valence changes in electrolytic reduction. In addition, it has been used for analyzing solutions as dilute as 10* molar, since the magnitude of the current increase at any particular cathode potential is proportional to the concentration of the ion being dischar ged at that potential.

Despite the wide application of the dropping mercury electrode to investigations involving cathodic phenomena, attempts to employ this electrode in study of anodic phenomena have been limited to those reactions whose oxidation potential is smaller than the potential at which the mercury itself is oxidized. Thus, in the field of anodic polarography, the dropping mercury electrode has the disadvantage of susceptibility to oxidation at a relatively low voltage of about +0.4, which seriously limits its application to this phase of polarographic analysis.

More recently, attempts have been made to extend the range of usefulness of the polarographic method by employing, in place of the dropping mercury electrode, the rotating platinum microelectrode. While it has been possible in some instances to use this latter electrode as an anode up to the potential at which oxygen is evolved, i. e., about +1.2 volts, the electrode has generally been of only limited applicability when so employed due to the lack of a renewable surface.

It is a major object of this invention to provide an electrode system which would enable the general study of polarographic oxidation phenomena in a range well be yond that attainable with the dropping mercury electrode. -A further object is the provision of an improved apparatus for carrying out polarographic analysis and other electrochemical studies. A still further object is to provide a method for investigating and analyzing solutions using the aforesaid apparatus.

The above and other objects which will be apparent to those skilled in the art are achieved in accordance with the present invention. Broadly, the instant invention comprises an electrode system resulting from the exposure of a constantarea of a continuously renewed, electrically conducting wire characterized by an electrolytically inert surface to a continuously renewed electrolytic solution under investigation. More specifically, the present invention comprises an electrode system achieved by drawing In the first compartment, the wire undergoes prepolarization treatment, which conditions its surface for electrochemical reaction with the test solution contained in the second active electrode compartment. In this compartthem, a constant area of the continuously moving wire is exposed to a continuouslyflowing solution of Y the test sample,the rate of flow of said solutionbeing less than the linear velocity of the wire passing therethrough. A low resistance reversible coupling electrode completes the electrical circuit of the cell system. When the above cell is used in place of the dropping mercury electrode, employed for conventional polarographic analysis, it has beenlfound that the voltage-range over which said analyam can be carried out may be appreciably extended in the investigation of'anodic phenomena. I

The'invention'may be more readily understood byreference to the attached drawing wherein Figure l'shows the apparatus of the invention in schematic form, 'Fig. 2

is a typical anodic polarogram obtained on hydroquinone in aqueous perchloric acid while Fig. 3 is an anodic polarogram obtained on phenyl-alpha-naphthalylamine in a solution of perchloric acid and isopropanol.

Referring more particularly to this figure, the measuring electrode is the section of wire exposed to the sample solution 11 in cell 12. The surface of the wire in contact with the sample solution is maintained constant by the geometry of the measuring cell and is continuously renewed as the wire is drawn through the cell. One extremity of the wire is coiled on a drum 13 and connected to a polarizing potentiometer 14. The other extremity of the wire is connected through an insulator link 15 to shaft 16 driven by motor 17 which serves to draw the wire through the cell. The cell is suitably constructed of a chemically and electrically resistant material such as polythene, ceramic, glass, or the like. The potentiometer 14 is connected through polarograph recorder 18 to a coupling electrode 19, which may be any low-resistance reversible electrode such as a silver, silver chloride, or calomel electrode, and is connected electrolytically with the sample solution, preferably through an inert porous diaphragm 20.

The solution in contact with the wire is continuously renewed as the sample flows downwardly through the cell, the rate of flow being regulated by exit capillary 21. In order to produce an electrochemically inert surface, a prepolarizing cell 22 is used. In this cell, the length of wire exposed is preferably greater than in the measuring cell. Another coupling electrode 23, which likewise may be any low-resistance reversible electrode, is connected, preferably through an auxiliary polarizing potentiometer 24, to the main polarizing potentiometer 14. Prepolarizing cell 22 is filled with the supporting electrolyte used in the analysis or other suitable liquid.

The cells are sealed by means of gaskets 25 of an elastic, chemically and electrically resistant material such as neoprene, which may be held in place by packing glands.

In operation of the electrode system, the travel of the wire is synchronized with the variable voltage applied by the polarizing potentiometer 14. The prepolarizing potentiometer 24 is regulated so that the Wire passing through the measuring cell 12 will be electrolytically inert; that is, the wire is placed in such a state that the current passing therethrough, due to causes other than electroreaction of the material undergoing investigation, is practically negligible. The prepolarizing voltage applied to the wire is synchronized with the travel of the wire so that, as each respective section of the wire arrives in the measuring cell, it will have been prepolarized to an extent suflicient to render it electrolytically inert toward the particular measuring voltage being applied to the measuring cell. Prepolarizing treatment is essential in placing the electrode in an inert condition and in removing surface films of active materials, and in counteracting strains, or chemical and mechanical variations in the electrode surface. The present polarizing unit functions so as to bring each section of the electrode to an inert state i. e., having a voltage substantially equal to the voltage to be applied in the measuring cell. In certain instances, it may be desirable to accelerate the prepolarizing treatment. This is accomplished by using a prepolarizing voltage of 0.1-0.2 of a volt greater than that being applied to the measuring electrode. In such instances, as will be seen from Figure 1 of the drawing, the prepolarizing voltage is the sum of the voltage supplied by polarizing potentiometer 14 and the voltage supplied by prepolarizing potentiometer 24, the latter serving to supplement the former to the extent of the desired 0.1-0.2 of a volt. The prepolarizing treatment may be used while measurements are being made. The prepolarizing treatment is not a substitute for chemical or mechanical cleaning treatments. These may be useful additional treatments. However, chemical and/or mechanical treatments alone are not sufficient to produce an electrolytically inert electrode.

The measuring electrode of this invention possesses the polarographic characteristic of a continuously renewable surface since a completely new electrode surface is continuously exposed to the sample solution. Also, a constant electrode surface area is in contact with the test solution at all times. This results from fixed dimensions of the electrode compartment. A constant thickness of diffusion layer on the electrode surface with respect to time is achieved by a regulated flow of sample solution through the measuring electrode compartment. An inert electrode surface is obtained by subjecting the wire electrode to the above-described prepolarizing treatment.

The method for obtaining the desired current-voltage curves employing the present apparatus is similar to the process wherein the conventional dropping mercury electrode is used, in that the voltage is increased and measurement of current is made. The current-voltage curves may be obtained manually as described above in connection with the dropping mercury electrode or a recording polarograph may be used in conjunction with the instant electrode system in a manner similar to that used in conventional polarographic analysis.

The apparatus of the present invention possesses four essential characteristics, namely:

(1) Continuous renewal of the electrode surface,

(2) Constant area of the exposed electrode surface,

(3) Continuous supply of solution to the exposed electrode surface, and

(4) An electrolytically inert electrode surface.

The present apparatus, possessing the above four enumerated features, constitutes an advantage over the conventional dropping mercury electrode, in that the area of electrode surface is constant for short-time intervals and no averaging of the current is necessary. In this regard, it may be noted that, with the dropping mercury electrode, the current at each value of the applied E. M. F. is not constant but oscillates between a minimum and a maximum value due to the periodic change in area as each mercury drop grows and falls. A further advantage of the instant electrode system is that the electrode surface can be made inert toward oxidation so that oxidation or anodic polarograms may be made at voltages beyond the range of the dropping mercury electrode. Thus, the range of the latter electrode is approximately +0.4 to 2.5 volts, whereas the instant electrode system affords investigation over an approximate voltage range of from +1.2 to about 2.5 volts.

By choice of suitable materials for the wire or by coating the wire with various substances, a wide range of electrode materials may be used. The specific action of various electrode materials can be used to advantage in many cases. The wire employed in the present electrode system is one having electrically conducting properties and usually is composed of an electrically conducting material such as copper, silver, aluminum, platinum, etc. Electrode wires of platinum and alloys thereof have been found to be particularly feasible in the instant apparatus.

The rate of flow of solution under investigation, contained in the measuring cell, is less than the linear velocity of the wire passing therethrough. In general, the relative velocity rates of the test solution and the wire have a ratio of between about 1:1 and about 1:100. Thus, the wire may suitably move at 2.5 mm./second and the corresponding solution flow rate may be 0.1 mm./ second.

The prepolarizing cell and the measuring and coupling electrodes of the instant electrode system are desirably contained in a block of a chemically resistant and electrically non-conducting material. In some instances, it may be desirable to separate the prepolarizing and measuring electrode cells, separately surrounding each with a chemically and electrically non-conducting material, and

maintaining an air space therebetween so as to eliminate electrical leakage across the gaskets between the two cells. It is further generally desirable to operate the electrode system under constant temperature conditions.

The following examples will serve to illustrate the apparatus and method of the invention without limiting the same:

Example 1 The electrode system, similar to that shown in Figure 1, was centered within a polythene plastic block drilled to contain three connecting compartments, one for the coupling electrode, another for the prepolarizing electrode, and a third for the measuring electrode surrounded by the test solution. The measuring electrode constituted a 5 mm. section of 0.006" (0.15 mm.) diameter wire composed of a 90 per cent platinumper cent iridium alloy. One extremity of this wire electrode was coiled on a small drum and connected to a recording polarograph by means of a flexible copper coil. The wire electrode was drawn through the prepolarizing and sample compartments at a rate of 2.5 mm. per second by a synchronous motor-operated shaft. The other extremity of the Wire was connected through a Teflon (polytetrafluoroethylene) insulator-link to a steel wire which, in turn, was coiled on the shaft. As the platinum alloy wire passed through the cell system, a 5 mm. segment of constant and renewable electrode surface was exposed to the sample solution. The area of the wire so exposed was 2.35 sq. mm. Entrance and exit passageways for the wire electrode were sealed by neoprene gaskets which were secured by packing glands. During operation, sample solution was permitted to continuously drain through the measuring cell at a rate of 0.1 ml. per minute.

The coupling electrode used to complete the electrical circuit of the cell system Was a silver-silver chloride electrode immersed in 1 normal aqueous lithium chloride solution. This solution was separated from the sample solution by a porous diaphragm. The prepolarizing unit consisted of a silver-silver chloride electrode, similar to the coupling electrode, and immersed in an inert supporting electrolyte contained in the prepolarizing compartment. This unit also included a No. 6 dry cell and a 500-ohm radio-type potentiometer employed in prepolarization of the electrode. 7

In operation, the measuring and prepolarizing electrode compartments were filled with supporting electrolyte. The wire electrode, before entering the prepolarizing compartment, was cleaned with an acid. Then, the polarograph and the synchronous motor, which draws the electrode through the cell system, were started simultaneously. Prepolarizing voltages were applied to the wire during passage through the prepolarizing compartment. A polarogram was then traced, which constituted the blank determination. Thereafter, the sample solution was introduced into the measuring cell and a polarogram of such solution was obtained.

A typical anodic polarogram obtained on 100 milligrams/liter of hydroquinone in 0.1 N aqueous perchloric acid, employing the above particular apparatus and procedure, is shown in Figure 2.

Example 2 An anodic polarogram obtained on 107 milligrams/liter of phenyl-alpha-naphthylamine in 0.2 N perchloric acid solution in 99 per cent isopropanol, employing the apparatus and procedure described in Example 1, is shown in Figure 3.

The data set forth in connection'with the above specific examples demonstrate the possibility of polarographically studying the oxidation characteristics of compounds in a voltage range well beyond that attainable with the conventional dropping mercury electrode.

Since the amount of test material corresponds to films of molecular dimensions, utilization of the instant electrode in studying the deposition and renewal of surface 6 films is indicated. It is contemplated that the greatest potential field of application for the described electrode system is in organic, anodic polarography. However, it is to be understood that the above description is merely illustrative of preferred embodiments of the invention, of which many variations may be made within the scope of the following claims by those skilled in the art without departing from the spirit thereof.

. I claim:

1. A method for making a polarographic determina tion, which comprises continuously drawing an electrically conducting wire at a constant rate through a prepolarizing zone and thereafter through a measuring cell containing an electrolytic test solution, applying a gradually increasing voltage to said wire as it passes through said prepolarizing zone, applying a substantially equal gradually increasing voltage to said wire as it passes through said measuring cell so that current conducted through the wire in the measuring cell due to causes other than electroreaction of material undergoing investigation in said test solution is practically negligible, continuously flowing said test solution through said measuring cell over a constant exposed surface of wire passing therethrough and recording the change in current of the wire in said measuring cell with change in applied voltage.

2. A method for making a polarographic determination, which comprises continuously drawing an electrically conducting wire at a constant rate through a prepolarizing zone and thereafter through a measuring cell containing an electrolytic test solution, applying a gradually increasing voltage to said wire as it passes through said prepolarizing zone, applying a gradually increasing voltage to said Wire as it passes through said measuring cell, maintaining a fixed difference between the voltage applied to said wire in the prepolarizing zone and the voltage applied to said wire in the measuring cell to the extent that the former exceeds the latter by approximately 0.1-0.2 of a volt, continuously flowing said test solution through said measuring cell over a constant exposed surface of wire passing therethrough and recording the change in current of the wire in said measuring cell with change in applied voltage.

3. An electrode system comprising a prepolarizing cell, a measuring cell, a pair of low resistance, reversible coupling electrodes, an electrically conducting wire, means for continuously drawing said wire at a constant rate successively through said prepolarizing cell and said measuring cell, means for continuously flowing an electrolytic solution through said measuring cell over a constant exposed surfacearea of said wire passing therethrough, means for electrolytically connecting said prepolarizing cell and said measuring cell separately to each of said coupling electrodes and means for applying voltage through said coupling electrodes to each of said cells.

4. An electrical circuit comprising a measuring cell, a prepolarizing cell, coupling electrodes, a polarizing potentiometer, a polarograph recorder, an auxiliary polarizing potentiometer and a continuously renewed electrically conducting wire passing through said prepolarizing cell and thereafter through said measuring cell, said wire having a voltage applied thereto during passage through said prepolarizing cell substantially equal to the voltage subsequently applied in said measuring cell and thereafter being brought into contact with a continuously renewed electrolytic solution contained in said measuring cell under conditions of constant exposed surface area of said wire to said solution, said potentiometer being connected through the polarograph recorder to a coupling electrode which, in turn, is electrolytically connected with the electrolytic solution contained in said measuring cell, and the prepolarizing cell being electrolytically connected to a second coupling electrode which, in turn, is connected through said auxiliary polarizing potentiometer to said polarizing potentiometer.

5. An electrode system adapted for polarographic analysis comprising an electrically conducting Wire, a prepolarizing cell, a. measuring cell, means for continuously drawing said wire at a constant rate successively through said prepolarizing cell and said measuring cell, means for applying voltage to said wire during passage thereof through said prepolarizing cell, means for applying voltage to said measuring cell and means for continuously flowing an electrolytic test solution through said measuring cell over a constant exposed surface area of said wire passing therethrough whereby current flowing through said wire in said measuring cell is attributable solely to electroreaction occurring in said test solution.

References Cited in the file of this patent UNITED STATES PATENTS 2,377,550 Hall June 5, 1945 5 FOREIGN PATENTS 723,185 Germany July 31, 1942 OTHER REFERENCES Analytical Chemistry, vol. 21 (1949), page 50 of 10 article by Lingane. 

1. A METHOD FOR MAKING A POLAROGRAPHIC DETERMINATION, WHICH COMPRISES CONTINUOUSLY DRAWING AN ELECTRICALLY CONDUCING WIRE AT A CONSTANT RATE THROUGH A PREPOLARIZING ZONE AND THEREAFTER THROUGH A MEASURING CELL CONTAINING AN ELECTROLYTIC TEST SOLUTION, APPLYING A GRADUALLY INCREASING VOLTAGE TO SAID WIRE AS IT PASSES THROUGH SAID PREPOLARIZING ZONE, APPLYING A SUBSTANTIALLY EQUAL GRADUALLY INCREASING VOLTAGE TO SAID WIRE AS IT PASSES THROUGH SAID MEASURING CELL SO THAT CURRENT CONDUCTED THROUGH THE WIRE IN THE MEASURING CELL DUE TO CAUSES OTHER THAN ELECTROREACTION OF MATERIAL UNDERGOING INVESTIGATION IN SAID TEST SOLUTION IS PRACTIALLY NEGLIGIBLE, CONTINUOUSLY FLOWING SAID TEST SOLUTION THROUGH SAID MEASURING CELL OVER A CONSTANT EXPOSED SURFACE OF WIRE PASSING THERETHROUGH AND RECORDING THE CHANGE IN CURRENT OF THE WIRE IN SAID MEASURING CELL WITH CHANGE IN APPLIED VOLTAGE.
 5. AN ELECTRODE SYSTEM ADAPTED FOR POLAROGRAPHIC ANALYSIS COMPRISING AN ELECTRICALLY CONDUCTING WIRE, A PREPOLARIZING CELL, A MEASURING CELL, MEANS FOR CONTINUOUSLY DRAWING SAID WIRE AT A CONSTANT RATE SUCCESSIVELY THROUGH SAID PREPOLARIZING CELL AND SAID MEASURING CELL, MEANS FOR APPLYING VOLTAGE TO SAID WIRE DURING PASSAGE THEREOF THROUGH SAID PREPOLARIZING CELL, MEANS FOR CONTINUOUSLY AGE TO SAID MEASURING CELL AND MEANS FOR CONTINUOUSLY FLOWING AN ELECTROLYTIC TEST SOLUTION THROUGH SAID MEASURING CELL OVER A CONSTANT EXPOSED SURFACE AREA OF SAID WIRE PASSING THERETHROUGH WHEREBY CURRENT FLOWING THROUGH SAID WIRE IN SAID MEASURING CELL IS ATTRIBUTABLE SOLELY TO ELECTROREACTION OCCURING IN SAID TEST SOLUTION. 